Mechanisms of inward transmembrane proton translocation

Using structural, functional and spectroscopic investigations of a bacterial inward proton-pumping rhodopsin, Kovalev et al. reveal its molecular mechanism and show how proton wires mediate ion selectivity and direct proton transport through cell membrane. Proton transport is indispensable for cell life. It is believed that molecular mechanisms of proton movement through different types of proton-conducting molecules have general universal features. However, elucidation of such mechanisms is a challenge. It requires true-atomic-resolution structures of all key proton-conducting states. Here we present a comprehensive function-structure study of a light-driven bacterial inward proton pump, xenorhodopsin, from Bacillus coahuilensis in all major proton-conducting states. The structures reveal that proton translocation is based on proton wires regulated by internal gates. The wires serve as both selectivity filters and translocation pathways for protons. The cumulative results suggest a general concept of proton translocation. We demonstrate the use of serial time-resolved crystallography at a synchrotron source with sub-millisecond resolution for rhodopsin studies, opening the door for principally new applications. The results might also be of interest for optogenetics since xenorhodopsins are the only alternative tools to fire neurons.

Automated summary

By studying a bacterial inward proton-pumping rhodopsin, researchers have uncovered its molecular mechanism and revealed how proton wires mediate ion selectivity and proton transport through cell membrane. Proton transport is essential for cell life and understanding the mechanisms behind it is a challenge. The study presents a comprehensive function-structure analysis of a light-driven bacterial inward proton pump, providing insights into proton translocation and the use of synchrotron source for rhodopsin studies. The results also have implications for optogenetics.